The present invention relates to an antiskid brake control device which not only prevents loss of directional stability and controllability of a motor vehicle, etc. due to locking of wheels caused at the time of braking but secures a minimum braking distance.
Conventionally, in antiskid brake control devices of this kind, owing to inevitable factors such as hysteresis of application of a braking force, delay in actuation of a hydraulic control device at the time of recovering the wheels exhibiting a locking trend, from locking and in the case where locking of the wheels has progressed fully, the driver desires to recover the wheels from locking as rapidly as possible. Thus, generally, at the time when the driver has considered that the wheels are recovered from locking, a hydraulic braking pressure is reduced excessively. Hence, in the case where the hydraulic braking pressure is increased immediately after the above described reduction of the hydraulic braking pressure, it will be ordinarily desirable that the hydraulic braking pressure is initially increased rapidly at a large rate in order to regain a necessary braking force as rapidly as possible and then, is slowly increased at a small rate after achievement of the necessary braking force in order to prevent the wheels as much as possible from being locked again.
Meanwhile, in a simplest antiskid control system, since a hydraulic control device controls the hydraulic braking pressure through changeover of only two modes, i.e. a pressure increase mode and a pressure reduction mode, it is impossible to change rate of increase of the hydraulic braking pressure as described above.
In order to solve the above described problem, an antiskid brake control device has been proposed in which a hydraulic pressure control device is provided with a pressure holding mode for holding the hydraulic braking pressure without reducing or increasing the hydraulic braking pressure so as to have three mode, i.e. the pressure increase mode, the pressure reduction mode and the pressure holding modei In this known antiskid brake control device, the hydraulic braking pressure is increased in accordance with a time period of reduction of the hydraulic braking pressure immediately after recovery of the wheels from locking and subsequently, holding of the hydraulic braking pressure for a relatively long time and increase of the hydraulic braking pressure for a relatively short time are repeated alternately such that the above described demand is satisfied.
However, the following problem arises in the prior art antiskid brake control device having the three modes, i.e. the pressure increase mode, the pressure reduction mode and the pressure holding mode, in which amount of rapid increase of the hydraulic braking pressure is determined in accordance with the time period of reduction of the hydraulic braking pressure required for recovering the wheels from locking, while increase, reduction and holding of the hydraulic braking pressure are subjected to open-loop control. Namely, when the motor vehicle is running on a slippery road surface, i.e. a road surface having a low coefficient of friction .mu., a force which is applied from the road surface to the locked wheels so as to recover the wheels from locking is small, so that a time period for reducing the hydraulic braking pressure becomes long. In this case, the hydraulic braking pressure is required to be increased again carefully for the wheels having exhibited a symptom of recovery of the wheels from locking such that amount of rapid increase of the hydraulic braking pressure is decreased.
On the other hand, in the case of a less slippery road surface, i.e. a road surface having a high coefficient of friction .mu., the wheels may be locked fully due to variations in time of detection of locking of the wheels and such instantaneous phenomena as passing of the wheels through puddles of the road surface and bouncing of the wheels off a bumpy road, so that a time period for reducing the hydraulic braking pressure becomes longer. Under these conditions, large amount of rapid increase of the hydraulic braking pressure is required to be employed for the wheels having recovered from locking. However, rate of reduction of the hydraulic braking pressure becomes usually larger and smaller as the hydraulic braking pressure is higher and lower, respectively due to the fact that quantity of oil flowing through a restriction passage formed in a solenoid valve is affected by difference in pressure between opposite ends of the restriction passage and owing to characteristics of stiffness of the hydraulic braking pressure. On the other hand, rate of increase of the hydraulic braking pressure does not change between high hydraulic braking pressure and low hydraulic braking pressure so large as in reduction of the hydraulic braking pressure.
Therefore, if, in the case of both the slippery road surface and the less slippery road surface, an identical time period for increasing the hydraulic braking pressure is at all times set for an identical time period for reducing the hydraulic braking pressure, the following inconveniences are incurred. Namely, in the case of the slippery road surface, the hydraulic braking pressure is low originally, so that reduction of the hydraulic braking pressure does not progress sufficiently and thus, the hydraulic braking pressure is increased excessively. On the other hand, in the case of the less slippery road surface, the hydraulic braking pressure is originally high, so that reduction of the hydraulic braking pressure progresses rapidly and thus, the hydraulic breaking pressure is not increased sufficiently.
FIG. 2 shows a characteristic curve of stiffness of the hydraulic braking pressure. As will be seen from FIG. 2, a large quantity of the oil is necessary for increasing the hydraulic braking pressure when the hydraulic braking pressure is low, while the hydraulic braking pressure varies greatly upon minute change of quantity of the oil when the hydraulic braking pressure becomes higher.
In order to deal with the above described drawback of the open-loop control, it may be considered that, at and after the second increase of the hydraulic braking pressure, a command of increasing the hydraulic braking pressure for a short time and a command of holding the hydraulic braking pressure for a relatively long time are repeated alternately but, at the first increase of the hydraulic braking pressure, feedback control is added such that either the hydraulic braking pressure is increased while an acceleration of the wheels is not less than a predetermined value or variations of the acceleration of the wheels are so controlled as to follow a preset target. However, in this case, the hydraulic braking pressure may be readily increased excessively due to hysteresis of braking, delay in actuation of the hydraulic control device, etc. In order to issue a command of increasing, reducing or holding the hydraulic braking pressure, in which hysteresis of braking, delay in actuation of the hydraulic control device, etc. are compensated for, a number of differentiation circuits and integration circuits are required to be provided because the system is not a simple linear model. Especially, in the case where the acceleration of the wheels is measured not in an analog manner but by amount of pulses, accuracy and stability of the differentiation system are not sufficient for such compensation.